Technical Field
The present invention relates to a method of controlling access of a wireless communication device, and more particularly, to a method of controlling access for high-efficiency communication of a wireless communication device in an energy harvesting network environment.
Description of Related Art
In an IoT network environment, IoT devices use a limited energy storage device such as a battery or a capacitor and consume energy in a data collection and transmission process. There is a problem that the IoT device cannot perform a continuous operation during a long period due to a limited energy storage device. Nowadays, as technology that can solve an operation limit problem due to a limited battery of an IoT device, energy harvesting technology has been in the spotlight. Energy harvesting technology is technology that generates electric energy from an energy source existing in a peripheral environment such as solar light, a heat, a pressure, and an electromagnetic wave. Because electric energy is generated through energy harvesting, the IoT device can continuously communicate even without battery exchange. In such an energy harvesting IoT network, the number of operable IoT devices changes according to a harvested energy amount and a consumed energy amount (i.e., according to an energy queue state of devices) and thus a method of adaptively managing a radio resource is required.
In an existing Framed Slotted ALOHA (F-ALOHA) protocol, every frame has a structure configured with the fixed number of slots and has a structure repeated on a time axis. The IoT device selects a random slot within a frame and transmits data at the selected slot. A length of a frame is generally represented with the number of slots constituting the frame. When a frame is terminated, slots constituting the frame may be classified into a success slot, a collision slot, and an idle slot. When only one IoT device transmits data at one slot, the one slot becomes a success slot, and when at least two IoT devices transmit data at one slot, the one slot becomes a collision slot. When no device transmits data at one slot, the one slot becomes an idle slot.
A frame of the F-ALOHA protocol is configured with a control slot and competition slots. The control slot includes a synchronization signal for synchronizing between an Access Point (AP) and an IoT device and ACK of an IoT device succeeded in data transmission at competition slots of a previous frame. The competition slots are used when IoT devices transmit data to an AP. The IoT device selects a random slot among competition slots within a frame and transmits data at the slot.
FIG. 1 is a diagram illustrating an operation example of an F-ALOHA protocol in a network configured with an AP and four IoT devices. In FIG. 1, B is a control slot in which the AP transmits a synchronization signal and ACK information to IoT devices. S means a success slot, C means a collision slot, and I means an idle slot. In FIG. 1, a frame length is fixed to 5. First, the AP transmits a synchronization signal at a control slot with the start of the frame and transmits an ACK signal to the IoT device succeeded in data transmission at a previous frame. At an (i−1)th frame, IoT device 1 selects a first competition slot to transmit data. Because the number of IoT device, having transmitted data at the first competition slot is one, the IoT device 1 succeeds in data transmission. IoT device 2 transmits and succeeds in data transmission at a fourth competition slot of the (i−1)th frame. IoT devices 3 and 4 simultaneously transmit data at a third competition slot of the (i−1)th frame and thus collision occurs. Because no IoT devices transmit data at the second competition slot, the second competition slot becomes an idle slot. The AP notifies the start of the frame through a control slot of an i-th frame and transmits ACK to the IoT devices 1 and 2 succeeded in data transmission at the (i−1)th frame. At the i-th frame, the entire four IoT devices select different competition slots and succeed in data transmission.
FIG. 2 is a diagram illustrating an operation example of an F-ALOHA protocol in an energy harvesting environment. It is assumed that a communication environment of FIG. 2 is a communication environment in which four energy harvesting IoT devices transmit data to one AP. In FIG. 2, E represents energy of an IoT device, and H represents an energy amount in which the IoT device harvests at an (i−1)th frame. Here, energy represents an energy amount charged at a battery of the IoT device, and it is assumed that an energy block is used as a basic unit and energy is configured with maximum five energy blocks. When the IoT device transmits data at an arbitrary frame, the IoT device transmits one data and uses one energy block. The IoT device may perform energy harvesting at every slot to increase energy thereof, and when the energy is larger than a specific threshold value Emin, the IoT device may transmit data. In FIG. 2, it is assumed that Emin=0.
At the (i−1)th frame of FIG. 2, the IoT devices 1 and 2 each individually select a first competition slot and a fourth competition slot and succeed in data transmission. As the IoT devices 1 and 2 each use one energy block, respective energies become 3 and 0. At the (i−1)th frame, as the IoT devices 3 and 4 simultaneously select the third competition slot, collision has occurred, and respective energies become 2 and 0. At a control slot of an i-th frame, the IoT devices update energy information including harvested energy at the (i−1)th frame. At the (i−1)th frame, the IoT devices 1 and 3 each acquire one energy block through energy harvesting and thus respective energies become 4 and 3. The IoT devices 2 and 4 do not succeed in energy harvesting at the (i−1)th frame and thus both energies thereof become 0. At the i-th frame, because the respective energies of IoT devices 2 and 4 do not exceed an energy threshold value (Emin=0), the IoT devices 2 and 4 do not transmit data but perform only energy harvesting. In the IoT devices 1 and 3, because respective energies thereof exceed Emin, the IoT devices 1 and 3 attempt data transmission at the i-th frame. The IoT device 1 and 3 each select a first competition slot and a third competition slot at the i-th frame and succeed in data transmission.
In the F-ALOHA protocol, because a frame length is fixed, there is a merit that a protocol is simply implemented. However, when a fixed frame length is used, as shown in FIG. 2, a problem occurs that efficiency of data transmission is deteriorated. For example, within one frame, when so many IoT devices attempt data transmission, compared with a frame length, collision occurs in most competition slots and thus communication between the IoT device and the AP is difficult, and when the small number of IoT devices transmit data to the AP, compared with a frame length, most competition slots become an idle slot to waste a radio resource.
When energy harvesting is available, the number of IoT devices that attempt data transmission varies according to energy of the IoT device. There is a drawback that the F-ALOHA protocol does not correspond to a change of the number of transmittable IoT devices. In FIG. 2, at the i-th frame, because the number of IoT devices that attempt data transmission is two and a frame length is 4, a situation occurs in which two competition slots are wasted. That is, when the number of IoT devices that attempt data transmission is smaller than a frame length, a slot resource is wasted, and in an environment in which the number of IoT devices that attempt transmission changes according to such an energy state, a resource efficiency problem of the F-ALOHA protocol is worsened. Therefore, in an IoT network in which energy harvesting is considered, it is inefficient to use the F-ALOHA protocol, a research on a Medium Access Control (MAC) protocol is required that can adaptively use a resource to the number of IoT devices varying according to an energy change.